1. Field of the Invention
The present invention relates to a radio communication system of a FM (frequency modulation) modulation/demodulation type and, more specifically, to a radio communication system adapting time division duplexing/frequency hopping.
2. Description of the Related Art
A general radio communication system performs communication using a fixed carrier wave upon communication. That is, after setting the transmission/reception frequency determined in a frequency setter, the communication is performed with the above set transmission/reception frequency until the communication is ended. In a radio communication system of frequency modulation/demodulation, which is a kind of the radio communication systems as described above, data is generated from a data source and input to a low pass filter and lowpass-filtered. Meanwhile, a frequency setter controls a transmission frequency combiner so as to convert the data of the preset transmission frequency. The transmission frequency combiner inputs the transmission frequency corresponding to the transmission frequency preset in an FM modulator, to the FM modulator in correspondence with the control of the frequency setter. At this time, the FM modulator receives the lowpass-filtered data and performs the frequency modulation according to the transmission frequency. Here, the frequency-modulated data is called the transmission data. The transmission data is input to a band pass filter and the bandpass-filtered transmission data is input to a power amplifier. The power amplifier amplifies the bandpass-filtered transmission data which is then again input to another band pass filter which bandpass-filters the amplified transmission frequency prior to being output to a duplexer. The duplexer radiates the bandpass-filtered transmission data through an antenna.
In the meanwhile, upon reception, a signal received from the antenna is input through the duplexer to a tuner and a tuning frequency of the tuner is provided as the same frequency as the frequency preset in the frequency setter. Also, the output of the tuner is input to a low noise amplifier which amplifies the signal and provides the amplified signal to a mixer. The mixer mixes the reception frequency preset in the frequency setter with the reception signal of the low noise amplifier and performs a typical down-conversion.
On the other hand, the output of the mixer is input to a band pass filter and forms a reception signal of the intermediate frequency level. The reception signal is input to another mixer and mixed with an intermediate frequency. The mixed signal is input to an FM demodulator and the output of the FM demodulator as a bandpass signal is input through a low pass filter to the data source.
In the radio communication system of the FM modulation/demodulation as mentioned above, the transmission/reception frequency used in the communication is fixed. Accordingly, it is difficult to perform the communication in the case that there is interference by an intentional or unintentional jammer. Further, since the communication is performed by using the fixed carrier wave, there is a problem in that a third party can easily hear it, intentionally or unintentionally.
The foregoing communication system uses FDD (frequency division duplexing). The FDD is used to discriminate, with certainty, the transmission frequency band and the reception frequency band. It is known to position a guide band between the transmission frequency band and the reception frequency band, in order to reduce the interference between the transmission frequency and the reception frequency. In accordance with this, it results in deterioration of frequency usage efficiency with limited radio wave source in case of the FDD. Also, it is required with the duplexer because of performing the communication with one antenna. Resultedly, the cost may be increased accordingly.
Furthermore, the radio communication system as stated above is affected by the intentional or unintentional jammer, thereby lowering the frequency usage efficiency. It is known to use frequency hopping in some spread spectrum communication systems to avoid intentional or unintentional jammer. Two examples of radio communication systems using frequency hopping are found in U.S. Pat. No. 4,694,466 to Joseph Kadin and entitled Time Sharing Frequency Synthesizer, and U.S. Pat. No. 4,760,586 to Shigeki Takeda and entitled Spread Spectrum Communication System.